Reconfigurable complex emulsions: Design, properties, and applications

Reconfigurable complex emulsions, which consist of multiphase droplets that can change morphology between encapsulated and Janus shapes, have become of recent fundamental and applied interest due to their unique stimuli-responsive characteristics. The newfound ability to dynamically change the structure and interfaces of droplets provides exciting opportunities for exploiting the properties and applications of fluids in ways not before possible, such as to create tunable lenses or droplet-based sensors. Droplet morphological reconfiguration, which is easily induced upon alteration of the balance of interfacial tensions, can be triggered in response to stimuli including pH, light, enzymes, temperature, and surfactants. This review describes recent advances involving reconfigurable complex droplet design, properties, and applications, highlighting both the opportunities and challenges associated with harnessing complex emulsions as responsive materials. We focus primarily on emulsions in which all droplet phases are immiscible with each other and the continuous phase, such as multiphase oil droplets dispersed in water or multiphase aqueous droplets dispersed in oil. The ability to manipulate the ordering of liquid interfaces in an emulsion while maintaining droplet stability has greatly enabled new directions for emulsion research and applications. Harnessing the dynamic structure and properties of reconfigurable complex emulsions presents a new frontier in the design of responsive materials relevant to optics, sensing, and active matter.